In power plant operations, regenerative heat exchangers such as air preheaters are used to recover sensible heat contained in a flue gas flow leaving the boiler to transfer it to combustion air, entering therein. In addition to preheating the combustion air, the recovered heat may be utilized for heating an air stream that may be utilized for drying coal for being pulverized and burning in a furnace.
Depending upon the fuel characteristics and overall system design, the air streams characteristics and qualities are developed. For example, in the power plants, such as in Oxygen-fired power plants for oxy-fired applications, generally a mixture of nearly pure Oxygen and recycled flue gas are used for combustion of fuel. In such applications, there are most likely chances of Oxygen getting passed towards a flue gas side due to static pressure differential between the recycled flue and flue gas streams. Such leakage is of a major concern because of high capital cost of an Air Separation Unit (ASU) incorporated in the Oxygen-fired power plants for separating Oxygen from air, and its high parasitic power consumption. To reduce such leakage, usages of pressurized sealing are quite in practice and may have generally been considered satisfactory for their intended purposes, but may be unsatisfactory to prevent residual amount of Oxygen turn over towards the flue gas side due to such leakage.